Download pn Junction Thin Film Solar Cells

Outline
• Solar radiation
9 Electromagnetic wave
9 Solar spectrum
MAE 493R/593V- Renewable Energy Devices
9 Solar global radiation
• Solar thermal energy
Solar Energy
9 Solar thermal collectors
9 Solar thermal power plants
• Photovoltaics (Solar cells)
9 P-n junction solar cells
9 Dye-sensitized solar cells
9 Organic solar cells
• CO2 capture and photoelectrochemical cells
http://www.flickr.com/photos/royal65/3167556443/
What are Photovoltaics (Solar Cells)?
What are Photovoltaics (Solar Cells)?
Photovoltaics is the direct conversion of light into electricity at
the atomic level
Photovoltaic Cell Type:
Heterojunction thin-film solar cell
• Silicon Si) and other thin-film silicon)
• Cadmium Telluride (CdTe)
• Copper indium gallium selenide (CIS or CIGS)
•
Dye-sensitized solar cell (DSSC)
•
Organic solar cells
Photo + voltaic = convert light to electricity
Image source: http://www.energy.ca.gov/distgen/equipment/photovoltaic/photovoltaic.html
Fermi level and energy band structure of solids
The inner core electronic configuration of Pd is :
Core electrons
Energy
valence
electrons
4d105s
Conduction band
Valence band
1s
2s
p-n Junction Thin
Film Solar Cells
5s
4d
3d
3p
Core level
3s
Energy band structure of solid
Viewpoint of electronic structure
Viewpoint of energy
Image source: Wikipedia
1
Energy band structure of solids
Organic molecules:
• The HOMO level is to organic
semiconductors and quantum
dots what the valence band is to
inorganic semiconductors.
• The same analogy exists
between the LUMO level and the
conduction band.
Fermi level & energy band structure of solids
"Fermi level" is the term used to describe the
top of the collection of electron energy levels at
absolute zero temperature.
The Fermi level is the surface of that sea at
absolute zero where no electrons will have
enough energy to rise above the surface.
Band gap, Egap, is the energy difference
between the valence band and the
conduction band
HOMO: highest occupied molecular orbital
LUMO: Lowest unoccupied molecular orbital
Band gap: energy difference between HOMO and LUMO
Energy band structure of solids
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/fermi.html1
Energy band structure of solids
Current: long-distance electron transport in solids:
Only the electrons in the conduction
band are delocalized, able to transport
for long distance
Only the electrons in the valence
band can jump to the conductions upon
a bias
The electrons in the core level are
localized, do not get involved in the
current flow
Conductance of electrons in solid
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html
Energy band structure of solids
Conductivity of solid materials:
Energy band structure of solids
Photo-excitation of semiconductor
(Ephoton = hc/λ)
When hv > Eg, electron-hole pair are generated
hv → h + + e −
photon
hole
electron
Eg
Under light irradiation, the electrons jump from the valence band to the
conduction band
The photo-generated electrons in the conduction band are delocalized, able
to transport for a long distance, forming electric current
Positive-charge holes are generated in the valence band when the electrons
leave
http://hyperphysics.phy-astr.gsu.edu/hbase/solids/band.html
The photo-generated holes in the valence band are delocalized, able to
transport for long distance, forming electric current
2
Energy band structure of solids
Energy band structure of solids
Photo-current in solid materials:
(Ephoton = hc/λ)
ZnO nanowire
Au electrode
3.2 eV
light
Photo-excitation of semiconductor
UV-Visible absorption spectra of
TiO2 nanoparticles, Nick Wu
Transmittance
Absorpbance:
Current response to light irradiation
Energy band structure of solids
Energy band structure of solids
Silicon
Extrinsic Semiconductor, n-type Doping
• Silicon is group IV element – with 4 electrons in their valence shell.
• When silicon atoms are brought together, each atom forms covalent
bond with 4 silicon atoms in a tetrahedron geometry.
• Doping silicon lattice with group V elements can creates extra electrons
in the conduction band — negative charge carriers (n-type), As- donor.
• In n-type semiconductors, the electrons are considered to be the majority
charge carrier
• Doping concentration /cm3 (1016/cm3 ~ 1/million).
By courtesy of Xiangfeng Duan
By courtesy of Xiangfeng Duan
Energy band structure of solids
Energy band structure of solids
Extrinsic Semiconductor, p-type doping
p-n Junction
Source: google image
• Doping silicon with group III elements can creates empty holes in the
conduction band — positive charge carriers (p-type), B-(acceptor).
• In p-type semiconductors, the holes are considered to be the majority
charge carrier
By courtesy of Xiangfeng Duan
• A p-n junction is a junction
formed by combining p-type
and n-type semiconductors
together in very close
contact.
• In p-n junction, the current
is only allowed to flow
along one direction from ptype to n-type materials.
http://www.tf.unikiel.de/matwis/amat/semi_en/kap_2/backbone/r2_2_4.html
3
Energy band structure of solids
p-n diode I-V characteristics
⎡ ⎛ eV ⎞ ⎤
⎟⎟ − 1⎥
I = I 0 ⎢exp⎜⎜
⎣ ⎝ nk BT ⎠ ⎦
where I0 is the reverse
saturation current, n is the
ideality factor which depends
on semiconductor material
and fabrication
characteristics (n = 1 – 2).
p-n Junction Thin Film Solar Cells
Principles - How p-n thin film solar cells work
¾ Consider p-n junction with
very narrow n-region.
¾ The illumination is through
the thin n-side.
¾The SCR (space-charge
region) extend mainly in pregion with built-in field E0.
By courtesy of Ken Durose
http://www.specmat.com
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Principles - How p-n thin film solar cells work
Principles - How p-n thin film solar cells work
n-type
junction
p-type
n-type
junction
Stage 1: before light illumination
¾ The bands are aligned
conduction band
Eg
Valence band
p-type
Stage 3: charge separation upon light
illumination
¾ Electrons flow to the lower energy
level
¾ Holes flow to the opposite direction
¾ Electron-hole pairs continue to be
generated
Valence band
Stage 4: build an open-voltage
¾ Electrons flow to the lower energy
level and build up on the n-side
¾ Holes build up on the p-side
¾ The p-n junction separate the
electrons and holes. The built-up
charge generate the open voltage
Stage 2: upon light illumination
¾ electrons and holes are
generated on both p and n sides
upon light illumination
http://www.soton.ac.uk/~solar/intro/tech6.htm
http://www.soton.ac.uk/~solar/intro/tech6.htm
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Principles - How p-n thin film solar cells work
Principles - How p-n thin film solar cells work
n-type junction p-type
Stage 5: current output
¾ current lows through the external
circuit a p-n junction solar cell is
connected to an external circuit
• No material is consumed.
• The process has no any moving components, which enables high reliability
and silent operation.
http://www.soton.ac.uk/~solar/intro/tech6.htm
Source: Images SI Inc.
4
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
• Therefore the existence of built-in field E0 is important to create
accumulated electrons in the n-side and holes in the p-side.
• Only those EHPs photogenerated within Le to the SCR can contribute to
the photovoltaic effect.
• For long wavelength photons Æ absorbed in the neutral p-side Æ no E
field Æ diffusion.
• Those photogenerated EHPs further away from SCR than Le are lost by
recombination.
„
EHPs
Minority carrier diffusion
length Le.
exp(−αx)
x
Le = 2 Deτ e
τe- recombination lifetime of
electron.
De- diffusion coefficient on
the p-side.
Lh
W
EHP: electron-hole pair
Le
Iph
Thus, it is important to have
the minority carrier
diffusion length Le as long
as possible. Æ By
choosing Si p-n junction
to be p-type which makes
electrons to be minority
carriers; the electron
diffuse length in Si is
greater the hole diffusion
length.
EHPs
exp(−αx)
x
Lh
W
Le
Iph
Source: University of South Alabama
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
• For long wavelengths, 1–1.2 μm, α is small Æ absorption depth 1/α
is typically greater than 100 μm. Æ Need a thick p-side and long
minority carrier diffusion length Le.
EHPs
„
„
absorption
coefficient
exp(−αx)
Thus, p-side is 200-500
μm and Le is shorter than
that.
x
Photons are absorbed and
recombined near the
crystal surface Æ losses.
Lh
W
Le
Iph
Source: University of South Alabama
• For EHPs photogenerated by short-wavelength photons absorbed in the
n-side, within diffusion length Lh, can reach SCL and swept across to
the p-side.
• The photogenerated of EHPs that contribute to the photovoltaic effect
occurs in a region of
Lh + W + Le
If the terminals are shorted
then the excess electrons on
the n-side can flow through
the external circuit to
neutralize the excess holes in
the p-side Æ this current is
called photocurrent.
EHPs
x
Lh
W
Le
Iph
Source: University of South Alabama
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Photovoltaic I-V Characteristics:
Photovoltaic I-V Characteristics:
• Consider an ideal p-n junction photovoltaic device connected to a
resistive load R.
• I and V define the convention for the direction of positive current and
positive voltage.
absorption
coefficient
exp(−αx)
Source: University of South Alabama
• If I is the light intensity, then the short circuit current is
I sc = − I ph = − KI
K is constant that depends on particular device
• The photocurrent does not depend on the voltage across the p-n junction,
because it always some internal field to drift the photogenerated EHP.
Light
Isc = –Iph
I
V
Iph
• If R is not short circuit Æ the positive voltage V appears across the p-n
junction as a result of the current passing through.
V=0
R
• If the load is short circuit Æ the only current in the circuit is due to
photogenerated (photocurrent),Iph.
Source: University of South Alabama
Source: University of South Alabama
5
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Photovoltaic I-V Characteristics:
Photovoltaic I-V Characteristics:
• The voltage across the load R (with opposite polarity) reduces the
built in potential V0 of the p-n junction and hence leads to
minority carrier injection and diffusion.
• Besides Iph, there is also a forward diode current Id in the circuit
which arises from the voltage developed across R.
• Since Id is due to the normal p-n junction behavior Æ diode
characteristics,
⎡ ⎛ eV
I d = I 0 ⎢exp⎜⎜
⎣⎢ ⎝ nk BT
⎡ ⎛ eV ⎞ ⎤
⎟⎟ − 1⎥
I = − I ph + I 0 ⎢exp⎜⎜
⎣ ⎝ nk BT ⎠ ⎦
„
„
I = Id − Iph
⎞ ⎤
⎟⎟ − 1⎥
⎠ ⎦⎥
Id
V
Iph
R
Source: University of South Alabama
I = Id − Iph
• The total current (solar cell current),
„
The I-V characteristics of a
typical Si solar cell in Figure.
Normal dark characteristics
being shifted down by
photocurrent Iph (short
circuit), which depend on
light intensity, I.
The open circuit voltage, Voc,
is given by the point where
the I-V curve cuts the V-axis
(I = 0), typically 0.4-0.6 V.
Id
V
Iph
R
I (mA)
20
Dark
Voc
0
V
0.2
Iph
Light
–20
Twice the light
Source: University of South Alabama
Photovoltaic I-V Characteristics (load line analysis)
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Photovoltaic I-V Characteristics:
Photovoltaic I-V Characteristics:
• When a solar cell drives a load R, R has the same voltage as the solar
cell but the current through it is in the opposite direction to the
convention that current flows from high to low potential.
• Or they can be found easily from load line construction.
• The current I’ and voltage V’ can be found by solving two previous
equations simultaneously Æ not trivial analytical procedure.
I (mA)
Light
V′
0
I
0.2
V
I
R
0
0.6
0.4
0.2
V
I-V for a solar cell under an
illumination of 600 Wm-2.
–10
Operating Point
I′
Isc= –Iph
• The load line cuts the solar cell characteristics at P. Point P
satisfies both equations Æ represent the operating point of the
circuit.
I (mA)
Voc
V′
Voc
Slope = – 1/R
–10
The Load Line for R = 30 ž
(I-V for the load)
P
–20
Source: University of South Alabama
p-n Junction Thin Film Solar Cells
Isc= –Iph
• The maximum power delivered to the load is Pout = I’V’ Æthe area bound
by I- and V-axes and the dashed lines.
FF =
V′
0
0.2
–10
Isc= –Iph
–20
0.4
Operating Point
I′
P
The Load Line for R = 30 ž
(I-V for the load)
Source: University of South Alabama
Metrics of the performance of solar cells
Photo-responsivity
defined as the photocurrent extracted from the
solar cell divided by the incident power of the light
at a certain wavelength.
External Quantum Efficiency
defined as the number of charges Ne
extracted at the electrodes divided by the
number of photons Nph of a certain wavelength
incident on the solar cell
V
I-V for a solar cell under an
illumination of 600 Wm-2.
Slope = – 1/R
P
–20
Voc
0.6
V
Operating Point
I′
I mVm
I scVoc
FF is a measure of the closeness of the
solar cell I-V curve to the rectangular shape
I (mA)
0.6
I-V for a solar cell under an
illumination of 600 Wm-2.
Slope = – 1/R
• Maximum power delivered Æ by changing R Æ max area when I’ = Im
and V’ = Vm.
FF range is 70 – 80%
0.4
Solar Cells
Fill Factor:
• The fill factor (FF),
0.6
0.4
The Load Line for R = 30 ž
(I-V for the load)
Power Conversion Efficiency
defined as the ratio of the electric power
output of the cell at the maximum power
point to the incident optical power.
Source: University of South Alabama
6
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Band gap effect on energy efficiency:
„
Band gap effect on energy efficiency:
Si has Eg = 1.1 eV Æ correspond to a threshold wavelength of
1.1 μm Æ The incident energy with wavelength > 1.1 μm is
then wasted (~ 25%).
• Efficiency:
η = (VocIscFF)/Pin
Voc ∝Eg,
2.5
Isc ∝ number of
absorbed photons
Black body radiation at 6000 K
Spectral 2.0
Intensity 1.5
dW cm-2 (μm)-1
or 1.0
kW m-2 (μm)-1
AM0
AM1.5
• Decrease Eg, absorb
more of the spectrum
hv > Eg
0.5
0
0
0.2 0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8 2.0
Wavelength (μm)
• But not without
sacrificing output
voltage
Source: University of South Alabama
p-n Junction Thin Film Solar Cells
By courtesy of Xiangfeng Duan
p-n Junction Thin Film Solar Cells
Effects of electron-hole recombination on energy efficiency:
Energy efficiency - Single crystalline versus polycrystalline :
• Photons are absorbed and recombined near the crystal surface Æ
losses Æ severely reduce efficiency.
• Crystal defects, crystal surface and interface contain high
concentration of recombination-center. The loss is ~ 40% due to the
e-h recombination.
Small Grain
and/or
Polycrystalline
Solids
Efficiency ∝ τ1/2
Large Grain
Single Crystals
• These combined effect bring the efficiency down to about 45%.
d
Long d
High τ
High Cost
Source: University of South Alabama
p-n Junction Thin Film Solar Cells
Energy Efficiency:
• For a given solar spectrum,
conversion efficiency depends on
the semiconductor material
properties and the device structure.
• Considering all losses, the
maximum electrical output power is
~21 % for a high efficiency Si solar
cell.
• Si-based solar cell efficiencies 18%
for polycrystalline and 22 – 24% for
single crystal devices.
Insufficient photon energy
hυ < Eg
× 0.59
Excessive photon energy
Near surface EHP recombination
hυ > Eg
× 0.95
Collection efficiency of photons
× 0.6
Voc ≈ (0.6Eg)/(ekB)
× 0.85
τ is recombination lifetime of electron
It decreases as grain size (and the cost) decreases
p-n Junction Thin Film Solar Cells
100% Incident radiation
× 0.74
d
Long d
Low τ
Lower Cost
First Generation– Single Junction Silicon Cells:
89.6% of 2007 Production
45.2% Single Crystal Si
42.2% Multi-crystal SI
• Limit efficiency 31%
• Single crystal silicon - 16-19%
efficiency
• Multi-crystal silicon - 14-15%
efficiency
http://en.wikipedia.org/wiki/Thin_film_solar_cell
FF ≈ 0.85
Overall efficiency
η ≈ 21%
Silicon cell average efficiency
Source: University of South Alabama
By courtesy of Xiangfeng Duan
7
p-n Junction Thin Film Solar Cells
p-n Junction Thin Film Solar Cells
Second Generation –Thin Film Cells:
CdTe 4.7% & copper indium gallium selenide (CIGS) 0.5% of 2007 Production
Third Generation – Multi-junction Cells
• Thin film cells use about 1% of the expensive semiconductors compared to
First Generation cells.
• CdTe : 8 – 11% efficiency (18% demonstrated)
• CIGS: 7-11% efficiency (20% demonstrated)
• Enhance poor electrical performance while maintaining low production costs.
• Current research is targeting conversion efficiencies of 30-60% while
retaining low cost materials and manufacturing techniques.
• Multi-junction cells: 30% efficiency (40-43% demonstrated)
Source: wikipedia
By courtesy of Xiangfeng Duan
By courtesy of Xiangfeng Duan
Dye-Sensitized Solar Cell (DSSC)
¾ DSSC is an electrochemical cell operated by sunlight.
¾Photoelectrochemical cells (PECs) are solar cells which generate electrical
energy from light.
¾ DSSC is a photoelectrochemical system based on a semiconductor formed
between a photo-sensitized anode and an electrolyte
¾ DSSC is also known as Grätzel cells
Each DSSC cell consists of:
ƒ electrolyte
ƒ metal cathode
ƒ semiconducting photo-anode
Dye-Sensitized Solar Cell
(DSSC)
Photoanode is composed
of titanium dioxide
nanoparticles, covered with
a molecular dye that
absorbs sunlight, like the
chlorophyll in green leaves
Image source: Wikipedia
Dye-Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cell (DSSC)
Operating principle of DSSC:
Operating principle of DSSC:
• Absorption of light occurs in a
dye absorbed on a nonporous
TiO2 layer.
• Charge separation occurs at the
interface between the dye and the
electron conducting TiO2.
• Electron transport: electrons
transport from TiO2 to the
transparent conducting oxide
electrode.
• Hole transport: diffusion of
iodide to the dye, which extract
electrons from the iodide and
oxidizes it to triiodide.
• Reduction of triiodide at the Ptelectrode, when the generated
electron is transferred through an
outer circuit.
http://www.thefullwiki.org/Dye-sensitized_solar_cells
Photo-anode
Cathode (Pt)
oxidation
electrolyte
reduction
http://www.thefullwiki.org/Dye-sensitized_solar_cells
dye
TiO2
By courtesy of Michael Graetzel
8
Dye-Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cell (DSSC)
Operating principle of DSSC: The operating cycle of redox reaction
Operating principle of DSSC:
light
light
Matthews et al. 1996
Dye-Sensitized Solar Cell (DSSC)
Left image courtesy of R. Chang
Dye-Sensitized Solar Cell (DSSC)
Configuration of DSSC:
Components of DSSC:
Sandwich-type: Working and counter electrode pressed together using a
polymer separator
Substrate: glass with transparent conducting oxide (TCO) such as fluoridedoped tin oxide (FTO) or indium-doped tin oxide (ITO) deposited on the
back of a (typically glass) plate as the contact electrode
• Transparent
• Electrically conductive
• Connections to load
By courtesy of Aldo DI CARLO
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Dye: organic semiconductor
- Artificial leaf
Artificial Plant with Leaves exhibited at EXPO 2005
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Dye: organic semiconductor- Artificial leaf
Structure of the ruthenium sensitizers RuL3 (yellow) cis-RuL2(NCS)2 (red) and
RuL’(NCS)3 (green)
Gratzel,Inorg. Chem. 2005, 44, 6841-6851
9
Dye-Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Dye: (also called molecular sensitizers), organic semiconductor, usually
Ruthenium complex molecules
• Band gap is small enough to adsorb visible light or infrared light
(Ephoton = hc/λ).
• Good match in band structure between dye and metal oxide
• Stable during light irradiation
• Must have carboxylate or phosphonategroups, Ligands are
chemisorbed to metal oxide semiconductor surface
dye
Components of DSSC:
Dye: organic semiconductor
UV-Visible absorption spectra of dye molecules
TiO2
Dye-Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Photoanode: metal oxide film
• Porosity >50%, allow the dye molecules to be infiltrated into the whole
photoanode
• Nanoparticles (~ 20nm diameter), high surface area to allow attach more
dye molecules
• TiO2: easy to synthesize, abundant, inexpensive
• Other semiconductor materials (ZnO)
Components of DSSC:
Photoanode: new material architecture
3.2 eV
TiO2 nanoparticles, Nick Wu
UV-Visible absorption spectra of
TiO2 nanoparticles, Nick Wu
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Photoanode: Aligned nanowire as photo-anode
TiO2 nanobelts, Nick Wu
TiO2 nanorod array, Nick Wu & Z.
Hong
Dye-Sensitized Solar Cell (DSSC)
The electron transport must be faster than recombination to ascertain
quantitative collection of charge carriers
¾ Nanowires provide a direct
path to the substrate for fast
charge transport.
¾ Faster transport can tolerate
faster recombination-other
redox couples can increase Voc
by ~300 mV.
¾ Aligned pores for facile pore
filling and direct path for hole
transport.
Jason B. Baxter
By courtesy of Michael Graetzel
10
Dye-Sensitized Solar Cell (DSSC)
The electron diffusion length in photoanode:
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Electrolyte: redox couple to reduce dye
• Usually iodide/tri-iodide couple
• Reduces dye after electron injection to TiO2
• Oxidized by contact with second electrode
I 3− + 2e ⇔ 3I −
™ The electron diffusion length exceeds
largely the film thickness
™ The film thickness is less than 30
micrometer
By courtesy of Michael Graetzel
Dye-Sensitized Solar Cell (DSSC)
Dye-Sensitized Solar Cell (DSSC)
Components of DSSC:
Counter electrode: glass with TCO,
catalyzed with platinum or carbon as catalyst
Typical performance of DSSC
maximum-power efficiency (ηmp) :
isc - the integral short circuit
photocurrent density
Voc - the open-circuit photovoltage
ηfill - the fill factor
Es - the incident solar irradiance
Photovoltaic performance of a state-ofthe-art DSSC : the I–V curve
measured under AM 1.5 standard test
condition
By courtesy of Gerko Oskam
Dye-Sensitized Solar Cell (DSSC)
Why DSSC?
MICHAEL GRÄTZEL, http://www.worldscibooks.com/etextbook/p217/p217_chap08.pdf
Dye-Sensitized Solar Cell (DSSC)
Challenges of DSSCs:
Current Solar Cells: Silicon (p-n junction)
¾ expensive
¾ difficult to produce
¾ framing/substrate –heavy, fragile
DSSCs
¾ cost effective –much less expensive
¾ can produce using layered coatings on glass
¾ may be able to produce on flexible substrates
¾ Can be lightweight (higher energy density)
¾ Good performance in diverse light conditions: high angle of incidence,
low intensity, partial shadowing
Lab efficiencies <12% and stagnating
™ Low red and near-IR absorption
™ Low extinction coefficient requires high surface area
™ Only I-/I3-redox couple has slow recombination kinetics, but it has
unnecessarily large overpotential
Stability and robustness
™ Liquid electrolyte is undesirable, but solid state hole conductors
give lower efficiency
™ 108 turnovers of dye required for 20 year lifetime
™ the electrolyte (I-/I3) is corrosive
11
Organic Solar Cells
by courtesy of Qing-Hua Xu
Operating principle:
Four processes:
Light absorption
Exciton Diffusion
Charge Transfer/separation
Charge Collection
Active layer
Donor (p-type)
Acceptor (n-type)
cathode
anode
http://www.chemphys.lu.se/res
earch/projects/teratransport/
Carbon Trust
Organic Solar Cells
Organic Solar Cells
Operating principle:
Light absorption
(1) Efficiency ηA> 50%
Operating principle- charge separation process
Exciton diffusion
(2) Efficiency ηED ~ 10%
Charge
collection
Optical absorption length ~ 100nm
• Narrow band width
• Jsc limitation
• UV instability
Exciton diffusion length ~ 5 nm
• Too short
• Thickness limitation
• Jsc limitation
Light
adsorption
By courtesy of Helen Gerardi
Organic Solar Cells
Organic Solar Cells
Overall energy efficiency:
Operating principle:
Charge Transfer
(3) Efficiency ηCT=100%
Charge Collection
(4) Efficiency ηCC = 100%
• Electric field dependent generation
• Voc limitation
• Electric field dependent mobility
• Dispersive transport
• Contact resistance
• Metal/organic issue
By courtesy of Helen Gerardi
Light absorption,
Exciton diffusion,
Charge transfer,
Charge collection,
Forrest, S. R. MRS Bull. 2005, 30, 28-32
> 50%
~ 10%
~ 100%
~ 100%
Typical current-voltage characteristics of
an organic solar cell (l.j.a.koster)
12
Organic Solar Cells
Organic Solar Cells
Overall energy efficiency of organic solar cells:
Overall energy efficiency:
Tobin Mark’s group: PNAS 2008; 105:2873‐2787
Actually typical efficiency
reported: 2~6%
Thomas Kietzke, Advances in OptoElectronics. doi:10.1155/2007/40285
Organic Solar Cells
Features of organic solar cells:
Thomas Kietzke, Advances in OptoElectronics. doi:10.1155/2007/40285
Organic Solar Cells
donor & acceptor molecules:
¾ Large optical band-gap (about 2 eV)
Usually absorb light from ultraviolet to blue (as the emission is in
visible region)
need the development of new material to absorb in red and near
infrared
acceptor
donor
¾ Generally poor charge-carrier mobility
As a result of the above two: almost insulator if not excited
¾ Relatively strong absorption coefficients (> 105 cm-1)
poly-3-hexylthiophene
donor
Copper PhthalocyanineZinc
acceptor
3,4,9,10-perylenetetracarboxylicbis-benzimidazole
Organic Solar Cells
organic donor-acceptor heterojunctions:
Organic Solar Cells
Thin-film organic solar cells:
ƒ Overall thickness of organic layers is usually < 200 nm
ƒ Individual layers are usually between 10-60 nm
ƒ Therefore they are very delicate, sensitive to any scratch
ƒ Need to be very clean: sensitive to dust
¾ Donor and acceptor are mixed so that the distance between any
absorbing site and the charge separation interface is less than the exciton
diffusion length.
¾ There should be percolated pathways for electron and holes to cathode
and anode respectively. Therefore organic solar cells are much more
sensitive to the nanoscale structure.
¾ These devices could be fabricated by co-deposition of donor and
acceptor pigments or solution casting donor acceptor blends
Yang, F.; Shtein, M.; Forrest, S. R., Nat. Mater. 2005, 4, 37-41
Source: Mir F. Salek
13
Organic Solar Cells
Organic Solar Cells
Thin film manufacturing techniques:
Spin-coating techniques:
(1) Evaporation: Suitable for small molecules
ƒ Sputtering
ƒ E-beam evaporation
ƒ Vapor deposition
ƒ Solution is dropped on the rotating
substrate
(2) Wet processing: Suitable for polymers
ƒ Spin coating
ƒ Screen printing
ƒ Inkjet printing
ƒ Doctor blading
ƒ By centrifugal force, solution spreads on
the substrate
ƒ Thin film properties depend on rotation
speed curve, solution, temperature, vapor
pressure of material…
Source: Mir F. Salek
Organic Solar Cells
Source: Mir F. Salek
Organic Solar Cells
Manufacturing techniques:
Advantages of organic solar cells
¾
¾
¾
¾
Relatively cheap in production and purification.
Materials can be tailored for the demand
Can be used on flexible substrate.
Can be shaped or tinted to suit architectural applications.
Challenges of organic solar cells:
¾ low efficiency
¾ low stability
¾ low strength compared to inorganic photovoltaic cells
P. Sommer-Larsen - Photovoltaics 16-Jan-09
Photovoltaic System to Utility Grid
Solar Cell Efficiency
A photovoltaic system that operates in parallel with and may
deliver power to an electrical production and distribution network.
Lawrence Kazmerski, National Renewable Energy Laboratory (NREL)
By courtesy of Jim Dunlop, NJATC Curriculum Specialist
14
Photovoltaic System to Utility Grid
Photovoltaic System to Utility Grid
By courtesy of Jim Dunlop, NJATC Curriculum Specialist
Photovoltaics Application
By courtesy of Jim Dunlop, NJATC Curriculum Specialist
Photovoltaics Application
Denver International Airport, 2 MW
Inter-Mountain Electric
G24i announced first deal to fuel mobile phone growth with revolutionary
solar technology-DSSC
Photovoltaics Application
Google Complex – Mountain
View, CA, 1.6 MW Installed by
Cuppertino Electric
Photovoltaics Market
Worldwide PV installed in history
Photovoltaic cells designed to resemble
wood roofing shingles, National Institute of
–Fox News
Standards and Technology
Source: Kyocera Solar, Inc 2007
15
Photovoltaics Market
Photovoltaics Market
SAN FRANCISCO, CALIFORNIA, March 17, 2010
Worldwide solar photovoltaic (PV) installations reached 6.43 GW in 2009
The PV industry generated $38 billion in global revenues in 2009, while
successfully raising more than $13.5 billion in equity and debt, up 8% on the
prior year
Worldwide PV Markets are Booming!
Worldwide PV installations will reach 22.2 GW in 2011, up from 16.0 GW in
2010, 6.43 GW in 2009
North American PV market predicted to double in 2011
Source: Solarbuzz Marketbuzz 2010 Report
Major PV Markets by Country in 2009
Photovoltaics Market
PV cost
The US is projected to see 2.1GW of PV installation in 2011.
Source: Suzanne Deffree, - EDN, 01/12/ 2011
Photovoltaics Market
Paul Maycock, Photovoltaic Energy Systems, Inc
The 2004-2009 period managed to encompass several significant events:
•Silicon feedstock shortage and significant high prices for this raw material
•Steady price increases from 2004 through 2008 for photovoltaic cells and
modules
•~50% price decrease in 2009 over 2008 for photovoltaic cells and modules
•Significant increase in market share for thin films, and in particular, CdTe
•The rise of the multi-megawatt (utility-scale) installation
•A global recession and the crisis in the financial sector that uncovered the a
virtual shell game in the trading of derivatives that exposed significant
housing market debt
•Constrained debt and equity markets
When it comes to non-silicon-based costs, however, the top company in China can
produce a module at a cost of USD 0.90 per watt, compared with around USD
1.50 per watt for European companies.
Source: Becky Stuart, PV Magzine, 12/272010
PV Installation- Life Cycle Saving
Environmental Benefits
Return on Investment (ROI):
Source: Kyocera Solar, Inc 2007
Source: Kyocera Solar, Inc 2007
16
History of Photovoltaic Cells
1839
1883
1888
1905
1946
1954
1980
1986
1991
The photovoltaic effect was first recognized in by French physicist A.
E. Becquerel.
The first solar cell was built, by Charles Fritts, who coated the
semiconductor selenium with an extremely thin layer of gold to form
the junctions. The device was only around 1% efficient.
Russian physicist Aleksandr Stoletov built the first photoelectric cell
Albert Einstein explained the photoelectric effect.
Russell Ohl patented the modern junction semiconductor solar cell in
which was discovered while working on the series of advances that
would lead to the transistor.
The silicon p-n junction photovoltaic cell was developed by The highly
efficient solar cell was first developed by Daryl Chapin, Calvin
Souther Fuller and Gerald Pearson at Bell Laboratories
First polymer based solar cells.
First organic solar cell with donor and acceptor was invented by
Tang.
A dye-sensitized solar cell (DSSC) was invented by Michael Grätzel
and Brian O'Regan at the École Polytechnique Fédérale de
Lausanne.
17